This invention relates to a time-division multiplex communication receiving apparatus, and more particularly to a time division multiplex communication receiving apparatus having an improved power reducing means.
Generally, in a time-division multiplex communication system, signals on a plurality of communication lines are transmitted through a single transmission line by using in common this transmission line in a manner that it is divided in terms of time. Among such communication systems there is known a system wherein from a key station the time-divided multiple information signals are transmitted; the receiving apparatus of each of a plurality of terminal offices receives said information signals; and in response to the received information signals each terminal office makes various operations, for example, delivers return signals or permits the generation of control signals for the other apparatuses.
In the time-division multiplex communication system, however, those of the time series-arranged information signals transmitted from the key station which are to be received by the receiving apparatus of the terminal office are arranged at specific time intervals. That is, since there exists a specific relationship in respect of time between the receiving apparatus and the information signals received therein, the receiving apparatus can extract the information signals necessary thereto in accordance with this specific time relationship.
Accordingly, to maintain the receiving apparatus always at a "receivable condition" for receiving the information signals sent from the key station to make the associated circuits operative results in a wasteful power consumption, which results in a remarkable decrease in the service life of receiving equipments, which results in an increase in the maintenance expenses of the receiving equipments such as batteries.
Particularly in the case where the above-mentioned time-division multiplex communication receiving apparatus is, for example, a portable radio receiver for use in a selective calling system, this receiver is desired to be compact, light and resistible to long use. However, a battery constituting the power source of that receiver is only stored with a specific amount of energy. Accordingly, where it is desired to increase the service life of the receiver on the basis of such specific or limited amount of electricity, the power consumption as average in the receiver should be made small.
For the above-mentioned reason, all sections of the receiver are kept operative over all time bands until the receiver is fully synchronized with the information signals sent over from the key station, whereas, after the synchronization with said information signals has been achieved, part of the receiver, for example, a radio reception amplification section is so designed that it ceases to operate so as to decrease the power consumption during a period in which it has no information signals to be supplied to itself.
Usually, however, such portable radio receiver is often moved from one place to another with a subscriber. Accordingly, in places such as an underground station yard it often happens that radio reception is made entirely impossible. In such a case, the receiver provided with a power reducing means as mentioned above fails to be synchronized, so that all sections of the receiver have to be kept always operative until the synchronization is achieved, resulting in a wasteful power consumption.